Doc: Add doc how to create token

[glensc]

Traceback (most recent call last):
  File "/Users/glen/scm/slack/slack-progressbar/pb1.py", line 12, in <module>
    pbar = sp.new()
  File "/Users/glen/scm/slack/slack-progressbar/.direnv/python-3.10.1/lib/python3.10/site-packages/slack_progress/__init__.py", line 18, in new
    res = self.slack.chat.post_message(self.channel, self._makebar(0),
  File "/Users/glen/scm/slack/slack-progressbar/.direnv/python-3.10.1/lib/python3.10/site-packages/slacker/__init__.py", line 499, in post_message
    return self.post('chat.postMessage',
  File "/Users/glen/scm/slack/slack-progressbar/.direnv/python-3.10.1/lib/python3.10/site-packages/slacker/__init__.py", line 124, in post
    return self._request(
  File "/Users/glen/scm/slack/slack-progressbar/.direnv/python-3.10.1/lib/python3.10/site-packages/slacker/__init__.py", line 102, in _request
    raise Error(response.error)
slacker.Error: missing_scope